As a transmission network for optical communication, a single-mode optical fiber having zero dispersion in a 1.3 .mu.m wavelength band is installed all over the world. Recently, there has been a tendency for the amount of communication information to dramatically increase with the development of the information-oriented society. With the increase of information, the wavelength division multiplex (WDM) transmission has been widely accepted in the field of communication and we have now entered the era of the wavelength division multiplex transmission. The wavelength division multiplex transmission is an optical transmission method suitable for high-capacity high-speed communication because the wavelength for optical communication is not one but is divided into a plurality of wavelengths so that a plurality of light signals are transmitted.
However, the existing transmission single-mode optical fiber having zero dispersion in the 1.3 .mu.m wavelength band is generally great in transmission loss and when the wavelength division multiplex communication is performed by use of the 1.3 .mu.m wavelength band, the wavelength region does not coincide with the 1.55 .mu.m wavelength band which is the gain band of a typical optical amplifier, so that the optical amplifier cannot be used. This is a hindrance to long-distance optical communication. For this reason, recently, the wavelength division multiplex communication in the 1.55 .mu.m wavelength band has been performed by use of the existing transmission single-mode optical fiber having zero dispersion in the 1.3 .mu.m wavelength band.
However, when optical communication is performed in the 1.55 .mu.m wavelength band by use of the transmission single-mode optical fiber having zero dispersion in the 1.3 .mu.m wavelength band, since the existing transmission single-mode optical fiber has positive dispersion and a positive dispersion slope in the 1.55 .mu.m wavelength band, the dispersion of the signal of each wavelength of the wavelength division multiplex transmission increases as the light signal propagates through the transmission single-mode optical fiber, so that signal separation is difficult at the receiving side. As a result, the quality of optical communication degrades and the reliability of optical communication is lost.
Therefore, recently, a dispersion compensating optical fiber has been developed in order to solve this problem. The dispersion compensating optical fiber has negative dispersion and by connecting the dispersion compensating optical fiber to the receiving side of the transmission single-mode optical fiber, the positive dispersion of the light signal propagating through the transmission single-mode optical fiber is abridged by the negative dispersion of the dispersion compensating optical fiber, so that the dispersion of the light signal is substantially close to zero when the light signal is received at the receiving side. Thus, by connecting the dispersion compensating optical fiber to the transmission single-mode optical fiber, it becomes possible to separate the light signals of wavelength division multiplex transmission at the receiving side and high-capacity high-speed communication of high quality is expected.
As examples of conventional dispersion compensating optical fibers of this type, for example, Japanese Laid-open Patent Publications Nos. Hei-6-11620 and Hei-7-261048 are known. Japanese Laid-open Patent Publication No. Hei-6-11620 proposes that the dispersion and the dispersion slope of the transmission single-mode optical fiber should be simultaneously compensated for. Japanese Laid-open Patent Publication No. Hei-7-261048 shows an example of a distribution of the refractive index of a dispersion compensating optical fiber from which high negative dispersion and a negative dispersion slope are obtained.
In the field of optical communication, for a distribution of the refractive index of the dispersion compensating optical fiber from which high negative dispersion and a negative dispersion slope are obtained, a W type as shown in FIG. 1 is generally proposed as a more suitable one than a matched type as shown in FIG. 3.